The Amazon is the most biodiverse place on the planet and houses treasures that have started to amaze the scientific world. The fossil of a 13 million-years-old lower hindlimb bone of a ground sloth bearing 46 tooth marks, found in Loreto region, opened a portal to the past and helps understand the evolution of ancestral ecosystems such as the diet of the giant caiman Purussaurus before reaching adulthood.
This story has been written since 2004, when French palaeontologist François Pujos, a specialized in the anatomy and evolution of sloths, was invited to a multidisciplinary scientific expedition to explore the geology, hydrology and palaeontology of the Napo River, from Ecuador to Peru.
Finding fossils in the Amazonia is "extremely difficult" because there is almost no place to look for. Most (if not all) of Amazonia is covered by a dense tropical forest. However, in recent years, several teams have discovered valuable evidence about the history of this ecosystem.
"Along the river within Peru, the expedition discovered several rock outcrops of lignite deposits of the Pebas Formation. These outcrops were full of bones," said Rodolfo Salas-Gismondi, paleontologist and researcher of the BioGeoSciences Lab at Lima-based Cayetano Heredia University.
Nevertheless, a tibia of a ground sloth that called François' attention, as he noticed that this bone had 46 tooth marks. In 2004, we knew little about the animals living 13-million-year ago in proto-Amazonian, so it was difficult to determine the identity of the perpetrator.
Looking for the perpetrator
The mystery remained unsolved until last year, but "we started to investigate among the potential criminals, and we found that tooth marks in the tibia match with the rostral anatomy and dentition of the top predator of the Pebas System, the giant caiman Purussaurus," Salas-Gismondi said.
Thirteen million years ago, the giant Miocene caiman Purussaurus fed upon ground sloths within proto-Amazonian swamps. Testimony of these agonistic interactions is provided by a shinbone (the tibia) of a ground sloth bearing 46 tooth marks.
Evidence suggests that the victim, which might have roamed the shores to eat plants, was gripped from the lower hind limb and did not survive the attack.
"At adult size, some Purussaurus species reached 10 meters long and probably ambushed solidly shielded turtles and enormous mammals. This discovery from Peruvian Amazonia reveals that prior to reaching its giant size, young Purussaurus might have captured small mammals of about the size of a capybara (a rodent)," he explained.
Prior to this discovery, there was almost no evidence on the agonistic interactions between the inhabitants of the proto-Amazonian ecosystems, the Peruvian paleontologist commented.
Largest non-marine predator
"The tibia discovered in the Peruvian Amazonia is the first one belonging to a mammal from those times bearing crocodylian tooth marks and therefore crucial for understanding the dynamic of ancient ecosystems. This an unusual snapshot of the feeding behavior of Purussaurus, the largest non-marine predator since the extinction of non-avian dinosaurs," he remarked.
For more than fifteen years, the outcrops of the Pebas Formation have been explored and their vertebrate fauna has been studied. Predators of that time in South America were giant flightless birds, marsupials and crocodylians.
"Bisected tooth marks are typical of crocodylians and, as we know, these animals were abundant in the Pebas deposits. Thirteen million years ago, crocodylians of the Pebas System included an array of six caimans and one long-snouted gharial," he stated.
Purussaurus possess robust, conical teeth with blunt tips and no other crocodylian was large enough to produce this kind of damage.
"Our estimations provide a perpetrator of about 4-meter-long, in other words, a youngster Purussaurus. Adult size Purussaurus individuals reached between seven to ten meters long," Salas-Gismondi said.
With about 10 meters of total body length, adult size Purussaurus individuals were the largest predators after the extinction of non-avian dinosaurs and might have fed upon huge solidly shielded turtles and enormous mammals. However, when they were young, they fed upon modest size prey, like ground sloths.
Changes in the diet
Modern crocodylians, like the American alligator or the Nile crocodile, show profound changes in their diet along their life, from hatchlings to adulthood. For example, when new-born they feed upon insects and spiders but when they reach four-meter-long, they can capture mammals and turtles.
These changes occur alongside with an amazing increase of bite force that has been recorded in about 800-fold from hatchling to a total body length of 3.71 m in the American alligator.
The bite force of an adult Purussaurus has been calculated in 7 tons, more than four times the strongest bite ever measured in the animal kingdom (the saltwater crocodile Crocodylus porosus with 1.6 tons of bite force). Thus, with this bite force, adult Purussaurus individuals were able to incorporate to their diet whatever, no matter the size or hardness.
Environment in the Amazonia
The Purussaurus and the ground sloth inhabited the Pebas System, a predominantly aquatic environment of lakes and swamps that flourished in north-western South America from 20 to 11 million years ago. Purussaurus probably lurked from the swamps and the ground sloth might have roamed the shores to eat plants.
This proto-Amazonian ecosystem occurred before the onset of the Amazon River System and was characterized by the extensive diversification and abundance of crocodylians and aquatic molluscs. Seven crocodylian species have been documented living sympatrically 13 million years ago, including three small ones that might have fed upon mollusks.
The top predator of this ecosystem was the giant caiman Purussaurus. All the paleontological evidence supporting knowledge about this ecosystem was recovered in the rocks of the Pebas Formation of the Peruvian Amazonia.
(END) JOT/RMB/MVB
Published: 8/26/2020